The capacitor is a general electric parts and is widely used in power source circuits or noise filters for digital circuits in various electric and electronic products. Capacitors are roughly classified into electrolytic capacitors and other capacitors (e.g., ceramic capacitors, film capacitors).
At present, various kinds of electrolytic capacitors are being used and examples thereof include aluminum electrolytic capacitors and wet tantalum electrolytic capacitors. Here, particularly excellent effects are expected from an aluminum electrolytic capacitor in the present invention and accordingly, the present invention is hereinafter described by referring to an aluminum electrolytic capacitor, however, the present invention is not limited to an aluminum electrolytic capacitor and can be widely applied to electrolytic capacitors in general.
For the electrode material of the electrolytic capacitor, a valve metal is used. In the case of an aluminum electrolytic capacitor, aluminum is used for the electrode material. The basic structure of the electrolytic capacitor takes a form (element) such that anode and cathode are prepared each by forming a predetermined amount of an oxide film as a dielectric material on the surface of an electrode (if desired, the surface area is increased by a treatment such as etching and thereby the electrostatic capacitance is controlled), these two electrodes are disposed to face each other and an electrolytic solution is held therebetween with an intervention of a separator (release paper). This electrolytic capacitor element is seal-packaged to complete an electrolytic capacitor. Some electrolytic capacitor elements have a coiled structure or have a stacked layer structure.
In the electrolytic capacitor as described above, the properties of the electrolytic solution are an important factor governing the performance of the electrolytic capacitor. Particularly, accompanying recent downsizing of the electrolytic capacitor, the anode or cathode foil used has a high etching magnification and the resistivity of the capacitor body is large. To cope with this, the electrolytic solution used therefor is always required to have low resistivity (specific resistance) and high electrical conductivity.
Heretofore, the electrolytic solution of the electrolytic capacitor was generally prepared by dissolving a carboxylic acid such as adipic acid and benzoic acid or an ammonium salt thereof as an electrolyte in a solvent consisting of ethylene glycol (EG) as the main solvent and water was added thereto to about 10 wt %. The thus-obtained electrolytic solution has a specific resistance of about 1.5 Ω·m (150 Ω·cm).
On the other hand, a capacitor is required to have a low impedance (Z) so as to satisfactorily provide a suitable performance. The impedance is determined by various factors. For example, the impedance decreases when the electrode area of the capacitor is increased and therefore, in a large-size capacitor, a low impedance is naturally obtained. Also, there is an approach of attaining a low impedance by improving the separator. However, and particularly in a small-size capacitor, the specific resistance of the electrolytic solution is a large factor governing the impedance.
In recent years, an electrolytic solution having a low specific resistance and using an aprotic organic solvent such as GBL (γ-butyrolactone) has been developed (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 62-145713, 62-145714 and 62-145715). However, the capacitor using the aprotic electrolytic solution is by far inferior in impedance to a solid capacitor using an electronic conductor having a specific resistance of 1.0 Ω·cm or less.
An aluminum electrolytic capacitor uses an electrolytic solution and therefore, is poor in low-temperature properties. In fact, the ratio Z (−40° C.)/Z (20° C.) of the impedance at −40° C. at 100 kHz to the impedance at 20° C. is about 40 and fairly large. Under these circumstances, an aluminum electrolytic capacitor having a low impedance, a low specific resistance and excellent low-temperature stability is required at present.
Water used as one portion of the solvent in the electrolytic solution of the aluminum electrolytic capacitor is chemically active with the aluminum constituting the anode or cathode foil and this causes a problem that water reacts with the anode or cathode foil to generate a hydrogen gas. As a result, the pressure inside the capacitor is increased, stress is imposed to the capacitor element, the coil structure is deformed or broken, the electrolytic solution is splashed outside, the safety vent is actuated, and the properties are seriously deteriorated. Conventionally, an attempt to absorb the generated hydrogen gas has been made so as to eliminate the problem of hydrogen gas generated in a load test or the like of the electrolytic capacitor. For example, Japanese Examined Patent Publication (Kokoku) No. 59-15374 discloses an electrolytic solution for driving an electrolytic capacitor, characterized in that a carboxylic acid and an ammonium salt of carboxylic acid are added to a solvent obtained by adding from 5 to 20 wt % of water to ethylene glycol and to the prepared buffer solution, from 0.05 to 3 wt % of p-nitrophenol is added to prepare the electrolytic solution. When this electrolytic solution is used, an electrolytic capacitor protected from the production of boehmite or the generation of hydrogen gas and improved in the low-temperature stability and working life properties can be provided.
Furthermore, Japanese Examined Patent Publication (Kokoku) No. 63-14862 discloses an electrolytic solution for driving an electrolytic capacitor, which can provide an excellent anticorrosive effect for cleaning with a halogenated hydrocarbon, characterized in that o-nitroanisole is added to an electrolytic solution obtained by dissolving an organic or inorganic acid of various types or a salt thereof as a solute in a solvent mainly comprising ethylene glycol. This patent publication states that the o-nitroanisole used as an anticorrosive has an activity of absorbing hydrogen gas and provides an effect of absorbing hydrogen gas generated from the inside of the electrolytic capacitor during use and thereby preventing an accidental safety-vent operation or a change in the electrostatic capacitance.
However, according to the studies by the present inventors, it is found that although the p-nitrophenol or o-nitroanisole can provide an effect of absorbing hydrogen gas, at an initial stage, in an electrolytic solution for driving an electrolytic capacitor, is commonly used and has a low water concentration, a sufficiently high effect of absorbing hydrogen gas cannot be obtained and cannot be maintained when the amount of water in the solvent of the electrolytic solution is 20 wt % or more or when the electrolytic capacitor is used over a long period of time in a high-temperature environment.
The present invention has been made to solve those problems in conventional techniques and an object of the present invention is to provide an electrolytic solution for driving an electrolytic capacitor, which ensures low impedance, has an excellent low-temperature stability represented by the ratio of impedance between low temperature and ordinary temperature, has a good working life property and has a capability of providing an excellent hydrogen gas-absorbing effect even when an electrolytic solution using a mixed solvent having a large water content ratio is used or when the electrolytic capacitor is used in a high-temperature environment. The object of the present invention includes providing an electrolytic capacitor using the electrolytic solution.
Another object of the present invention is to provide an electrolytic capacitor using a driving electrolytic solution having a solvent composition with 30 wt % or more thereof being water, wherein a solvent-soluble nitro compound or nitroso compound is contained in the capacitor element.